We and our collaborators have previously measured the p53 mutation level in nontumorous human liver tissue. The data are consistent with the hypothesis that p53 mutation load may indicate previous carcinogen exposure and identify individuals at increased cancer risk. We are again using the most sensitive, RFLP/PCR based, genotypic mutation analysis system available thus far, to measure the mutation load at codons 247 and 248 of the p53 gene in nontumorous colon tissue from donors with colon cancer, with cancer prone conditions, e.g., ulcerative colitis, and normal donors. We detected G:C to A:T transitions in those nontumorous samples which were obtained from the donors where tumors also have G:C to A:T transitions. These transitions were observed with several-fold higher absolute mutation frequencies that the control samples from normal donors. Ulcerative colitis samples showed several- fold higher absolute mutation frequency of G:C to A:T transition than in infants and normal adults. These results indicate that the nontumorous colon tissue in the cancer patients with p53 codon 248 mutations and the patient with ulcerative colitis carry a higher p53 mutation load at codon 248 that those from noncancer donors without known colon diseases. Nitric oxide synthase-2 activity was increased in the inflamed versus non-inflamed colonic regions from patients with ulcerative colitis. Microsatellite instability was also observed in the inflamed regions and was positively correlated with p53 mutation load. These results are consistent with genomic instability associated with oxyradical overload from increased NOS2 expression.